Influence of the aerodynamic drag on the motion of interplanetary ejecta

A simple semi‐empirical model for the motion of interplanetary ejecta is proposed to advance the prediction of their arrival times at Earth. It is considered that the driving force and the gravity are much smaller than the aerodynamic drag force. The interaction with the ambient solar wind is modeled using a simple expression for the acceleration = −γ(υ− w ), where w = w ( R ) is the distance‐dependent solar wind speed. It is assumed that the coefficient γ decreases with the heliocentric distance as γ = α R −β , where α and β are constants. The equation of motion is integrated numerically to relate the Earth transit time and the associated in situ velocity with the velocity of coronal mass ejection. The results reproduce well the observations in the whole velocity range of interest. The model values are compared with some other models in which the interplanetary acceleration is not velocity dependent, as well as with the model where the drag acceleration is quadratic in velocity = −γ 2 (υ − w)∣υ − w∣.